A serosurvey for measles antibody was performed in 1,870 in Massachusetts high school students. All of the students had been immunized around their 1st year of life. Surprisingly, only 84% proved to be immune (positive) by the hemagglutination-inhibition test. However, using the sensitive plaque reduction test developed in this laboratory, three reactivity groups could be established: (1) Those negative for measles antibody (titer <1:4) comprised 1.4% of the cohort; (2) Approximately 10% had antibody levels <1:120 found previously to be nonprotective upon exposure to wild virus; and (3) The remainder had protective antibody levels >1:120. Students with negative or non-protective PRN titers were revaccinated with measles vaccine. Those with negative PRN titers developed an IgM response. Their GMT rose from <4 to 2810. At the six-year serological follow-up completed this year all were shown to maintain protective antibody levels (GMT 740, range 260-4,279). Those with low, antibody titers (<120) also gave a significant booster response from GMT of 54 to 1,320. They tested negative for I&M. Six years later only 60% of them had protective antibody levels, and the GMT was 134 (range 23-620), significantly lower than in the group of revaccinated primary vaccine failures (P<0.01). A review of the immunization history has shown that 50% of students with negative or non-protective titers received their first measles vaccination before their first year of life. These findings underline the importance of delaying measles immunization beyond the 1st year of life. They also suggest that a second dose of vaccine will not only reduced or eliminate primary vaccine failures but provide protective antibody levels to many who lost these levels after initial successful immunization.